Many kinds of valve seats including one made of an iron-based sintered alloy have hitherto been used in internal combustion engines such as an automobile engine, and studies have been made as to wear and abrasion resistance of the valve seats.
In general, an engine using a kind of liquid fuels such as gasoline and gas oil is advantageous to reduction of the wear and abrasion of the valve seat, because of maintenance of high lubricity between a valve and the valve seat through the fuel and combustion products including carbon. To the contrary, an operation of an engine using a kind of gaseous fuels such as natural gas involves metallic surfaces of the valve seat and the valve in a direct contact with each other, because an amount of combustion products is small in comparison with a case where the liquid fuel is used, and hence tends to develop the wear and abrasion, resulting in occurrence of a flow caused by plastic deformation and an adhesive wear and abrasion.
As to a method to improve the wear and abrasion resistance of the valve seat, there is known that hard particles such as Fe-Mo particles or Fe-W particles are dispersed in a matrix of the valve seat. However, when the wear and abrasion resistance of the valve seat is intended to be improved by increasing an amount of the hard particles, the valve which is a counterpart used in combination therewith is liable to be worn and/or abraded.
There have been disclosed some valve seats having an excellent wear and abrasion resistance and a small attacking property against the counterpart. For example, Japanese Patent Application Laid Open (KOKAI) No. HEI 5-43913 discloses a valve seat of iron-based sintered alloy formed by the method in which carbide-dispersed type and/or intermetallic compound-dispersed type hard particles having a Micro Vickers hardness in a range of 500-1800 are dispersed in an amount of 5-25 weight % in the matrix of iron-based sintered alloy, and the shape of the hard particle is made globular. Besides, Japanese Patent Application Laid Open (KOKAI) No. HEI 5-43998 discloses another valve seat of iron-based sintered alloy formed by the method in which carbide-dispersed type and/or intermetallic compound-dispersed type hard particles having a Micro Vickers hardness in a range of 500-1800 are dispersed in an amount of 5-25 weight % in the matrix of iron-based sintered alloy to form a base member of the valve seat, and thus formed base member is infiltrated with copper or copper alloy. In the aforesaid publications, however, there is no investigation regarding a countermeasure in case where the valve seat is brought into direct contact with a metallic surface of a counterpart, as in the engine using the gaseous fuel.